1. Field of the Invention
The present invention relates to an optical head and an optical disk apparatus for optical recording or reproduction of information on an optical disk.
2. Related Art of the Invention
An objective lens used for an optical head has been designed in consideration of the substrate thickness of an optical disk. Therefore, if the objective lens is used for an optical disk having a substrate thickness different from the design value, spherical aberration occurs and the convergence performance of the lens is lowered, thereby making recording and reproduction difficult. Conventionally, all of compact disks (CD), video disks and magneto-optical disks for data have a substrate thickness of 1.2 mm. Therefore, only one kind of objective lens can be used, and recording and reproduction can be accomplished on different kinds of optical disks by using only one optical head.
In the case of recently standardized digital video disks (DVD), the numerical aperture of the objective lens is increased, and the wavelength of the light source is shortened.
When the numerical aperture is increased, optical resolving power is raised, and the frequency band for recording and reproduction can be widened. However, if the optical disk is tilted, a problem of increased comatic aberration is caused. This problem is unavoidable since the problem is caused when the optical disk itself is warped, when the turntable wobbles, or even when dirt is present between the optical disk and the turntable. Comatic aberration due to the tilt is proportional to the third power of the numerical aperture and the substrate thickness. Therefore, even when the substrate thickness of an optical disks is unchanged, if the numerical aperture is raised, the aberration is increased, and the convergence performance is not raised. In the case of DVD, in order not to increase the comatic aberration even when the numerical aperture of the objective lens is increased, the substrate thickness of the optical disk is made smaller up to 0.6 mm, thereby reducing such an adverse effect. When the substrate thickness of the optical disk is made smaller as described above, however, nothing can be recorded and reproduced on the above-mentioned conventional thick optical disk by using an objective lens produced for the thin optical disk, whereby no compatibility can be maintained between the conventional optical disk and the thin optical disk.
Furthermore, in the case that a short wavelength of 650 nm is used, the numerical aperture of the objective lens can be increased, and the optical resolving power can be raised, whereby the frequency band for recording or reproduction can be widened. However, if the conventional optical disk having been set to be usable at a wavelength of 780 to 830 nm is played back by using a semiconductor laser with a shorter wavelength (650 nm), a problem of being unable to obtain any sufficient reproduction signal or control signal is caused due to differences in the reflectance, absorbance and the like of the recording surface. This problem is significant in CD-R, for example, which has been standardized as a rewritable CD. Originally, the CD-R has been standardized so that its reflectance is 65% or more at 775 to 820 nm. However, the reflectance drops extremely at wavelengths other than the standard range, and the absorbance increases. At a wavelength of about 650 nm, the reflectance may decrease to about one-eighth and the absorbance may increase about eight times in comparison with the standard values. In such a case, even when reproduction is attempted by applying the standard power, recorded data may be erased by the absorption of light, instead of being reproduced.
In order to solve these problems, a method using two light sources, such as that shown in FIGS. 11 to 16, has been devised conventionally. FIGS. 11, 12, 13 and 14 are views showing the configuration of a conventional optical head in accordance with this method. FIGS. 11 and 13 show the case when a high-density optical disk 8 having a substrate thickness of 0.6 mm is played back, and FIGS. 12 and 14 show the case when an optical disk 13 having a substrate thickness of 1.2 mm is played back. FIG. 13 is a perspective view of FIG. 11, and FIG. 14 is a perspective view of FIG. 12. FIG. 15 is a view showing the configuration of the module of the conventional optical head, and FIGS. 16a, 16b and 16c are graphs showing the characteristics of an optical film used for the head. FIG. 16a shows the transmittance of the film at an incident angle of 45-4.6.degree.; FIG. 16b shows the transmittance at an incident angle of 45.degree.; and FIG. 16c shows the transmittance at an incident angle of 45+4.6.degree.. The module mentioned in the present specification refers to an integrated combination of at least a semiconductor laser and a photo-detector.
Referring to FIG. 11, a light beam 42 with a wavelength of 650 nm emitted from the semiconductor laser 41a of a first module 41 passes through a hologram 41c, and enters a compound prism 43. Since the polarization direction of the light beam 42 is the longitudinal direction of the module 41 just as in the case of a second module 51 shown in FIG. 15, the light beam 42 enters the compound prism 43 as s-polarized light. An optical film 44 having the characteristics shown in FIGS. 16a, 16b and 16c is formed on the junction surface of the compound prism 43. Since the light beam 42 is used within a spread angle in the range of -7 to +7.degree., the compound prism 43 is configured so that the light beam 42 inside the glass of the compound prism 43 passes through the optical film 44 at least in the range of 45 -4.6 to 45+4.6.degree.. Therefore, since the s-polarized light passes through in the angle range, the light beam 42 entering as diverging light passes through the compound prism 43, and is condensed by a condenser lens 45 to become a nearly parallel lightbeam. The condensed light beam 42 is reflected by a mirror 46, passes through an aperture limitation means 47, and is focused on an image formation point p by an objective lens 48 to form a light spot 49 on the recording surface of the optical disk 8. This aperture limitation means 47 is configured so as to allow light with a wavelength of 650 nm to pass through totally, and so as to allow light with a wavelength of 780 nm to pass through only the internal portion thereof, which corresponds to a numerical aperture of 0.45. Furthermore, the objective lens 48 has a numerical aperture of 0.6, and is optimally designed for an optical disk having a substrate thickness of 0.6 mm. Therefore, the light beam 42 with a wavelength of 650 nm is focused in accordance with the numerical aperture of 0.6.
Next, a light beam 50 reflected by the optical disk 8 passes through the objective lens 48 and the aperture limitation means 47 again, is reflected by the mirror 46, and condensed by the condenser lens 45, and then enters the compound prism 43. In addition, the light beam 50 passes through the compound prism 43 and enters the first module 41. After entering the first module 41, the light beam 50 is diffracted by the hologram 41c, and enters a photo-detector 41b. The photo-detector 41b is configured to detect a focus control signal for making the objective lens 48 follow the recording surface by using the so-called SSD (spot size detection) method and to detect a tracking control signal for making the objective lens 48 follow the track on the track surface by using the phase contrast method.
Furthermore, the second module 51 is provided with a semiconductor laser 51a with a wavelength of 780 nm. Referring to FIG. 12, a light beam 52 with a wavelength of 780 nm emitted from the second module 51 passes through a hologram 51c, and enters the compound prism 43. Since the polarization direction of the light beam 52 is the longitudinal direction of the second module as shown in FIG. 15, the light beam 52 enters the compound prism 43 as s-polarized light. Therefore, the light beam 52 is reflected by the optical film 44 having the characteristics shown in FIGS. 16a, 16b and 16c, and condensed by the condenser lens 45 to become slightly diverging light. After reflected by the mirror 46, the light beam 52 passes through only the internal portion of the aperture limitation means 47, which corresponds to a numerical aperture of 0.45. The light beam then enters the objective lens 48, and is focused on an image formation point p' to form a light spot 53 on the recording surface of the optical disk 13. By limiting the aperture, the numerical aperture is set to 0.45, whereby this configuration can be made compatible with the optical disk 13 having a substrate thickness of 1.2 mm, such as CD.
A light beam 54 reflected by the optical disk 13 passes through the objective lens 48, the aperture limitation means 47 and the mirror 46 again, and is condensed by the condenser lens 45, and then enters the compound prism 43. Most of the light beam 54 is reflected and enters the second module 51. After entering the second module 51, the light beam 54 is diffracted by the hologram 51c, and enters a photo-detector 51b. The photo-detector 51b is configured to detect a focus control signal for making the objective lens 48 follow the recording surface by using the SSD method and to detect a tracking control signal for making the objective lens 48 follow the track on the track surface by using the push-pull method. Although the three-beam method is generally used to detect the tracking control signal for CD, the push-pull method is used in the conventional example in order to simplify explanations.
In the case that the high-density optical disk 8 compatible with a wavelength of 650 nm is played back by using the above-mentioned optical system, the semiconductor laser 41a is lit, its light beam is focused on the optical disk 8, and a light beam reflected by the optical disk 8 is received by the photo-detector 41b, whereby reproduction and control signals can be obtained. In the case that the optical disk 13 compatible with a wavelength of 780 nm is played back, the semiconductor laser 51a is lit, its light beam is focused on the optical disk 13, and a light beam reflected by the optical disk 13 is received by the photo-detector 51b, whereby reproduction and control signals can be obtained. In this way, the two types of optical disks 8 and 13 being different in thickness and wavelength compatibility can be played back.
In the above-mentioned optical heads the light beam 42 is diverging light and has a spread angle in the range of about -7 to +7.degree.. The spread angle in the range of -7 to +7.degree. in the air corresponds to a spread angle in the range of -4.6 to +4.6.degree. in glass. Therefore, the optical film 44 is required to have characteristics of allowing s-polarized light with a wavelength of 650 nm to pass through in the range of 45-4.6.degree. to 45+4.6.degree. and also allowing s-polarized light with a wavelength of 780 nm to be reflected, thereby requiring the characteristics shown in FIGS. 16a, 16b and 16c. These characteristics can be used only for s-polarized light. Even if wavelength characteristics are optimized, the positional relationship in the optical head cannot be arranged so that p-polarized light enters the compound prism 43.
When explained again, in the above-mentioned conventional art, the light beam emitted from the second module 51 for CD playback enters the compound prism 43 as s-polarized light. The polarization direction (direction of electric field) of the light beam is the longitudinal direction of the second module 51 as shown in FIG. 15. Therefore, the module of the optical head is forced to be arranged as shown in FIG. 11 because of the wavelength characteristics of the optical film 44 shown in FIGS. 16a, 16b and 16c.
In other words, the components of the optical head, that is, the module 41, the module 51, the prism 43, the optical film 44, the condenser lens 45, the mirror 46, the aperture limitation means 47 and the objective lens 48, are forced to be arranged on a plane perpendicular to the surface of the optical disk 8. As a result, the compound prism 43 and the second module (for CD) 51 are forced to be arranged in the thickness direction (L) of the optical head, whereby the entire thickness of the optical head is made larger.
Furthermore, in the case that the internal structure of the second module 51 is modified so that its polarization direction is the width direction of the module, the compound prism 43 and the second module 51 can be arranged on a plane parallel to the optical disk 8. However, the minor axis direction of the intensity distribution of the light beam emitted from the semiconductor laser 51a becomes the direction of the track, thereby lowering focusing performance in this direction, and thus being undesirable to reproduction. Moreover, modifying the internal structure of the module causes a problem in production, and producing a new module for CD causes a problem in cost.
An optical film having characteristics shown in FIGS. 5a, 5b and 5c has been developed, which has a transmittance of substantially 0% for both p-polarized light and s-polarized light in a specified wavelength range, and has a transmittance of substantially 100% for both p-polarized light and s-polarized light in another specified wavelength range. By using this optical film, an optical system can be configured regardless of the polarization direction from the light source. However, if this optical film is used for the diverging light in the configuration of the above-mentioned conventional optical head, the polarized light separation width (.DELTA.H in FIG. 5) between p-polarized light and s-polarized light becomes larger than the difference between two wavelengths being used (650 nm and 780 nm in the case of the conventional example), thereby making it difficult to configure an optical system regardless of the polarization direction from the light source.